In a mobile communication system, each mobile terminal such as a mobile phone is enabled to continue a wireless communication over a wide area by handover or by switching from one access point to another while the mobile terminal is in movement. In such a mobile communication system, the communication quality may be deteriorated at or near the boundary of the cover area (the range of wireless communication) of each access point, and the communication may be temporarily interrupted during the process of the handover. In particular, it is essential for systems that transmit video signals to take appropriate measures against such problems as the amount of info nation that is to be transmitted is greater as compared to systems for transmitting speech signals.
In the field of video communication systems, it is known to divide each still image frame of a video signal into a plurality of blocks, classify the blocks into blocks of interest and blocks of non-interest according to the importance of each block and reduce the amounts of codes preferentially for blocks of non-interest when the amounts of codes are required to be reduced so that at least the essential part of the video signal may be retrieved at a high quality even when the quality of communication is deteriorated. See Patent Document 1.
It is also known to provide each mobile terminal with a pair of transmission/reception units for communicating with the base stations, and cause one of the transmission/reception units which is not engaged in the communication with the handover source base station to exchange the control signal required for the handover with the handover target base station so that the time period of interruption in the communication at the time of the handover may be minimized. See Patent Document 2.